The NSIDC press release for the 4th September charts the continuing decline of Arctic sea ice, showing the August mean extent historical series, with 2007 dropping like a stone towards the bottom of the graph. “Don’t care,” is the response of some, “look at the trend for the Antarctic; it’s going up. It’s just natural variations.”
There are a lot of reasons why this idea is wrong-headed, but it gave the old man a thought.
How much sea ice is there at both poles put together? Is there any observable trend in the dual-hemisphere sea ice extent?
Doing a full time series for every month since 1979 (the satellite era) would take ages, so is there a shortcut? September and March are the two months where the sea ice generally reaches its respective maximum and minimum mean monthly extent in both hemispheres (how’s that for alliteration?) .
So I have extracted the mean monthly sea ice extent for these two months, for 1979-2006, from the NSIDC’s readily available dataset. Adding the NH and SH means together gives us a ‘global’ mean monthly extent, at the periods when the metric reaches its extremes at both poles (nominally). Then I summed the two separate months. Note here that there is more ice around in September (24-27 million km2) than there is in March (17.9-21 Mkm2). Also note that the range is greater in the SH than the NH.
What happened?
(Sorry about the clumsy link; I can’t get the graphics insert sorted)
Oh. This was a bit of a surprise. I actually expected there to be little or no observable trend. But there is one. The trend is noticeably downwards.
All of this is an exercise in playing with the numbers, of course, but it should put one idea to bed: even though the sea ice measurements in the Antarctic show a small positive anomaly in the modern record, it is not enough to offset the large negative anomaly in the Arctic. Put simply; globally, there appears to be less ice than there used to be. Now why would that be?
More later.
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September 5, 2007 at 1:40 pm
William
I’d have expected the Arctic trend to dominate, it being rather larger
September 5, 2007 at 1:49 pm
fergusbrown
You and I might have expected this, but a few people don’t get the point. Doesn’t matter how you measure it, the ice cube is shrinking.
btw; I should have written about that Schulte piece before linking you to it, shouldn’t I; then my blog might have got a mention in Nature…
Love the Mooney picture!
September 5, 2007 at 9:16 pm
Don Fontaine
Moving averages of the sort you use here are ugly. This is because they distort the data by including some data points more often than others when all the points should have equal weights (frequency of use in computing the average). The 1979 data point contributes to only one 10 period moving average, the one plotted between 1988 and 1989. The 1980 data point contributes to two 10 period moving averages, that between 1988 and 1989 and the one between 1989 and 1990. Some of the central points contribute to 10 moving averages, ten times the weighting of the 1979 point. This seems very unfair and would distort the overall mean. Is there some weighting algorithm I’m missing?
Don’t you think that plotting the moving average as a trailing average distorts the time scale? The first 10 period moving average is plotted midway between 1988 and 1989. Since this point is the average of the 1979, 1980, 1981, …, 1988 points, it makes more sense to me to plot it on the 1984 tic mark. And likewise for the subsequent averages. The final 10 period moving average, plotted between the 2006 and 2007 tic marks would end up on the 2002 tic mark.
The ice always melts fastest at the end. higher Surface area to volume ratio, higher rate of heat transfer.
Rather than combining arctic and antarctic, I want to difference the arctic extent and arctic area for the same time period. This might give some indication of how the cracks and polynyas (?) are changing.
September 5, 2007 at 9:53 pm
fergusbrown
It’s not even necessary, Don; I was just playing around. Doesn’t matter how you plot it, it’s clear how the trend will run; I just used a fairly familiar way of doing it, that’s all. Take any trend line out if you want and just look at the rest of the graph.
I don’t get your point about the ice always melting fastest near the end; are you suggesting that this is an entirely predictable consequence of successive loss being logarithmically effected by what precedes it? I don’t know how the interseasonal swings would effect this, but I suspect they would nullify the effect. What does that suggest about next year’s ice, or 2012?
There’s a site somewhere which does the extent/area calculations in neat little boxes, but I can’t remember where. There was, i remember, a huge amount of variability, which iIthink was dependent on the time of year (not surprising when you think about it).
Thanks for visiting. all are welcome in the cave.
September 5, 2007 at 10:29 pm
Don Fontaine
The difference between extent and area is cool but I don’t know how to send the excel plot. Very regular periodicity. There is a downward shift on Aug 87 that suggests a shift in at least one of the two data sets. July is always the max prior to the shift, after the shift May is almost always the max. This suggests that the most area for cracks and polynyas are in July or May.
September 5, 2007 at 11:04 pm
fergusbrown
When you say a ‘downward shift’, do you mean that the difference between area & extent decreases? You’re not very clear. It sounds like you’re describing the Antarctic, but you don’t specify; can you be a bit clearer?
One thing to consider is the possibility that the accuracy of the available data changed in 1987: a new system online with better resolution, or a second method of calculating the gaps in the ice, could have affected the difference data. Can’t see how it would change the dates of maximum difference, though.
Sorry I can’t advise you on sending the excel sheet, except to say that wordpress doesn’t seem to like these files; it’s easier to try to save the graph as a jpeg & post an address to it, or something…
September 6, 2007 at 1:32 pm
Don Fontaine
When you say a ‘downward shift’, do you mean that the difference between area & extent decreases? Yes
Arctic, not Antarctic, from NSIDC ftp site. The files I used are named like N_08_area.txt, for August from 1979 to 2007. They have both extent and area, I just subtracted the two numbers to get the last column. Sorry for sending all the data but it really does make a nice graph. I also added the date colum in the middle to make plotting in excel easier.
There is a jump in the year over year area around Aug 87 that causes the downward shift.
year mo data_type region extent area ext-areaGoddard N Nov-78 12.02 8.95 3. Goddard N Dec-78 14.12 10.85 3. Goddard N Jan-79 15.54 12.33 3. Goddard N Feb-79 16.31 13.11 3.2
1979 3 Goddard N Mar-79 16.44 13.13 3. Goddard N Apr-79 15.46 12.43 3. Goddard N May-79 14.06 11.04 3. Goddard N Jun-79 12.59 9.28 3. Goddard N Jul-79 10.47 6.59 3. Goddard N Aug-79 8.15 4.96 3. Goddard N Sep-79 7.2 4.53 2. Goddard N Oct-79 9.39 6.11 3. Goddard N Nov-79 11.16 8.29 2. Goddard N Dec-79 13.54 10.55 2. Goddard N Jan-80 14.96 11.85 3. Goddard N Feb-80 15.98 12.82 3. Goddard N Mar-80 16.13 12.92 3. Goddard N Apr-80 15.49 12.45 3. Goddard N May-80 14.04 10.88 3. Goddard N Jun-80 12.31 8.93 3. Goddard N Jul-80 10.39 6.45 3. Goddard N Aug-80 8.04 4.84 3.2
1980 9 Goddard N Sep-80 7.85 4.83 3. Goddard N Oct-80 9.46 6.42 3. Goddard N Nov-80 11.69 8.64 3. Goddard N Dec-80 13.72 10.71 3. Goddard N Jan-81 15.03 11.82 3. Goddard N Feb-81 15.65 12.45 3.2
1981 3 Goddard N Mar-81 15.61 12.62 2. Goddard N Apr-81 15.12 12.16 2. Goddard N May-81 13.9 10.93 2. Goddard N Jun-81 12.57 8.96 3. Goddard N Jul-81 10.62 6.23 4. Goddard N Aug-81 7.86 4.38 3. Goddard N Sep-81 7.25 4.38 2. Goddard N Oct-81 9.19 6. Goddard N Nov-81 11.17 8.23 2. Goddard N Dec-81 13.74 10.47 3. Goddard N Jan-82 15.26 12.11 3. Goddard N Feb-82 16.06 12.79 3. Goddard N Mar-82 16.15 12.99 3. Goddard N Apr-82 15.57 12.55 3. Goddard N May-82 14.17 11.1 3. Goddard N Jun-82 12.69 9.4 3. Goddard N Jul-82 10.75 6.79 3. Goddard N Aug-82 8.26 4.92 3. Goddard N Sep-82 7.45 4.38 3. Goddard N Oct-82 9.98 6.6 3. Goddard N Nov-82 11.91 8.97 2. Goddard N Dec-82 13.83 10.81 3. Goddard N Jan-83 15.1 11.92 3. Goddard N Feb-83 16.02 12.86 3. Goddard N Mar-83 16.1 12.84 3. Goddard N Apr-83 15.3 12.12 3. Goddard N May-83 13.54 10.8 2. Goddard N Jun-83 12.35 9.2 3. Goddard N Jul-83 10.91 6.79 4. Goddard N Aug-83 8.36 4.88 3. Goddard N Sep-83 7.52 4.64 2. Goddard N Oct-83 9.64 6.7 2. Goddard N Nov-83 11.64 8.91 2. Goddard N Dec-83 13.44 10.56 2. Goddard N Jan-84 14.6 11. Goddard N Feb-84 15.32 12.17 3. Goddard N Mar-84 15.62 12.48 3. Goddard N Apr-84 15.15 12.1 3. Goddard N May-84 13.68 10.81 2. Goddard N Jun-84 12.2 9.02 3. Goddard N Jul-84 10.15 6.19 3. Goddard N Aug-84 7.87 4.59 3. Goddard N Sep-84 7.17 4.05 3. Goddard N Oct-84 8.84 5. Goddard N Nov-84 11.29 8.15 3. Goddard N Dec-84 13.18 10.19 2. Goddard N Jan-85 14.85 11.6 3. Goddard N Feb-85 15.67 12.39 3. Goddard N Mar-85 16.06 12.66 3.4
1985 4 Goddard N Apr-85 15.34 12.46 2. Goddard N May-85 14.23 11.19 3. Goddard N Jun-85 12.4 8.88 3. Goddard N Jul-85 10.09 5.89 4.2
1985 8 Goddard N Aug-85 7.46 4.51 2. Goddard N Sep-85 6.93 4.17 2. Goddard N Oct-85 8.88 6.03 2. Goddard N Nov-85 11.39 8.35 3. Goddard N Dec-85 13.19 10.15 3. Goddard N Jan-86 15.02 11.79 3. Goddard N Feb-86 15.89 12.53 3. Goddard N Mar-86 16.08 12.65 3. Goddard N Apr-86 15.15 11.95 3.2
1986 5 Goddard N May-86 13.52 10.63 2. Goddard N Jun-86 12.1 8.93 3. Goddard N Jul-86 10.47 6.53 3. Goddard N Aug-86 8.01 4.92 3. Goddard N Sep-86 7.54 4.66 2. Goddard N Oct-86 9.89 6.77 3. Goddard N Nov-86 11.78 8.81 2. Goddard N Dec-86 13.4 10.46 2. Goddard N Jan-87 15.2 11.81 3. Goddard N Feb-87 16.11 12.93 3. Goddard N Mar-87 15.95 12.75 3.2
1987 4 Goddard N Apr-87 15.33 12.27 3. Goddard N May-87 13.81 10.91 2.9
1987 6 Goddard N Jun-87 12.57 9.32 3. Goddard N Jul-87 9.98 6.84 3. Goddard N Aug-87 7.69 5.35 2. Goddard N Sep-87 7.48 5.6 1. Goddard N Oct-87 9.29 7.44 1. Goddard N Nov-87 11.52 9.67 1. Goddard N Dec-87 15.61 13.49 2. Goddard N Jan-88 16.13 13.84 2. Goddard N Feb-88 15.21 13.11 2.1
1988 5 Goddard N Mar-88 13.69 11.35 2. Goddard N Apr-88 12.02 9.62 2.4
1988 7 Goddard N May-88 10.04 6.93 3. Goddard N Jun-88 7.9 5.34 2. Goddard N Jul-88 7.49 5.31 2. Goddard N Aug-88 9.47 7. Goddard N Sep-88 11.69 9.86 1. Goddard N Oct-88 13.78 11.94 1. Goddard N Nov-88 15.12 13.11 2. Goddard N Dec-88 15.56 13.28 2. Goddard N Jan-89 15.52 13.14 2. Goddard N Feb-89 14.44 12.26 2. Goddard N Mar-89 12.98 11.3 1. Goddard N Apr-89 12.31 9.9 2. Goddard N May-89 10.38 7.24 3. Goddard N Jun-89 7.92 5.48 2. Goddard N Jul-89 7.04 4.81 2. Goddard N Aug-89 9.52 6.9 2. Goddard N Sep-89 11.5 9.38 2. Goddard N Oct-89 13.47 11.53 1. Goddard N Nov-89 14.95 12.72 2. Goddard N Dec-89 15.56 13.33 2. Goddard N Jan-90 15.88 13.44 2. Goddard N Feb-90 14.68 12.16 2. Goddard N Mar-90 13.3 10.84 2. Goddard N Apr-90 11.68 9.12 2. Goddard N May-90 9.62 6.44 3. Goddard N Jun-90 6.82 4.92 1.9
1990 9 Goddard N Jul-90 6.24 4.5 1. Goddard N Aug-90 9.35 6.67 2. Goddard N Sep-90 11.31 9.58 1. Goddard N Oct-90 13.27 11.4 1. Goddard N Nov-90 14.46 12.49 1. Goddard N Dec-90 15.26 13.15 2. Goddard N Jan-91 15.5 13.35 2. Goddard N Feb-91 14.93 12.79 2. Goddard N Mar-91 13.51 11.44 2. Goddard N Apr-91 12.23 9.6 2. Goddard N May-91 9.68 6.66 3. Goddard N Jun-91 7.4 5.14 2. Goddard N Jul-91 6.55 4.46 2. Goddard N Aug-91 9.16 6.7 2. Goddard N Sep-91 11.12 9.19 1. Goddard N Oct-91 13.17 11.19 1. Goddard N Nov-91 14.72 12.54 2. Goddard N Dec-91 15.5 13.51 1. Goddard N Jan-92 15.47 13.41 2. Goddard N Feb-92 14.7 12.76 1. Goddard N Mar-92 13.25 11.37 1. Goddard N Apr-92 12.13 9.89 2. Goddard N May-92 10.61 7.14 3. Goddard N Jun-92 7.86 5.35 2. Goddard N Jul-92 7.55 5.37 2. Goddard N Aug-92 9.6 7.69 1. Goddard N Sep-92 11.87 9.69 2. Goddard N Oct-92 13.45 11.66 1. Goddard N Nov-92 15.08 12.85 2. Goddard N Dec-92 15.73 13.54 2. Goddard N Jan-93 15.88 13.71 2. Goddard N Feb-93 15.18 12.95 2. Goddard N Mar-93 13.54 11.35 2. Goddard N Apr-93 11.99 9.2 2. Goddard N May-93 9.66 6.17 3. Goddard N Jun-93 7.29 4.65 2. Goddard N Jul-93 6.5 4.52 1. Goddard N Aug-93 9.19 6.98 2. Goddard N Sep-93 11.73 9.49 2. Goddard N Oct-93 13.52 11.41 2. Goddard N Nov-93 14.82 12.8 2. Goddard N Dec-93 15.61 13.46 2. Goddard N Jan-94 15.58 13.47 2. Goddard N Feb-94 14.95 12.84 2. Goddard N Mar-94 13.73 11.6 2. Goddard N Apr-94 12.1 9.62 2. Goddard N May-94 10.22 6.85 3. Goddard N Jun-94 7.61 5.1 2. Goddard N Jul-94 7.18 5.08 2.1
1994 10 Goddard N Aug-94 9.48 7.17 2. Goddard N Sep-94 11.3 9.64 1. Goddard N Oct-94 13.53 11.43 2.1
1995 1 Goddard N Nov-94 14.62 12.72 1.9
1995 2 Goddard N Dec-94 15.24 13.3 1. Goddard N Jan-95 15.32 13.27 2. Goddard N Feb-95 14.59 12.32 2. Goddard N Mar-95 13.04 10.76 2. Goddard N Apr-95 11.55 8.86 2. Goddard N May-95 9.15 6.05 3.1
1995 8 Goddard N Jun-95 6.69 4.61 2. Goddard N Jul-95 6.13 4.38 1. Goddard N Aug-95 8.94 5.91 3. Goddard N Sep-95 10.97 8.95 2. Goddard N Oct-95 12.98 11.02 1. Goddard N Nov-95 14.21 12.07 2. Goddard N Dec-95 15.17 12.9 2. Goddard N Jan-96 15.12 12.83 2. Goddard N Feb-96 14.22 12.23 1. Goddard N Mar-96 13.06 11.22 1. Goddard N Apr-96 12.1 9.78 2. Goddard N May-96 10.36 7. Goddard N Jun-96 8.17 5.65 2. Goddard N Jul-96 7.88 5.58 2. Goddard N Aug-96 9.39 7.5 1. Goddard N Sep-96 10.56 8.7 1. Goddard N Oct-96 13.14 10.96 2. Goddard N Nov-96 14.47 12.3 2. Goddard N Dec-96 15.51 13.36 2. Goddard N Jan-97 15.58 13.24 2. Goddard N Feb-97 14.59 12.48 2. Goddard N Mar-97 13.32 11.17 2. Goddard N Apr-97 11.91 9.14 2. Goddard N May-97 9.59 6.41 3. Goddard N Jun-97 7.3 5.03 2. Goddard N Jul-97 6.74 4.84 1. Goddard N Aug-97 8.76 6.39 2. Goddard N Sep-97 10.91 9 1. Goddard N Oct-97 13.29 11.19 2.1
1998 1 Goddard N Nov-97 14.81 12.73 2. Goddard N Dec-97 15.77 13.7 2. Goddard N Jan-98 15.66 13.5 2. Goddard N Feb-98 14.89 12.76 2. Goddard N Mar-98 13.8 11.35 2. Goddard N Apr-98 11.85 9.11 2. Goddard N May-98 9.62 6.38 3. Goddard N Jun-98 7.49 4.61 2. Goddard N Jul-98 6.56 4.24 2. Goddard N Aug-98 8.85 6.65 2. Goddard N Sep-98 10.75 8.82 1. Goddard N Oct-98 13.26 10.85 2. Goddard N Nov-98 14.47 12.54 1. Goddard N Dec-98 15.37 13.38 1. Goddard N Jan-99 15.4 13.47 1. Goddard N Feb-99 15.13 13.08 2. Goddard N Mar-99 13.86 11.61 2. Goddard N Apr-99 12.1 9.18 2. Goddard N May-99 9.59 6.49 3.1
1999 8 Goddard N Jun-99 7.38 4.67 2. Goddard N Jul-99 6.24 4.22 2. Goddard N Aug-99 9.1 6.77 2. Goddard N Sep-99 10.99 9.21 1. Goddard N Oct-99 12.88 10.93 1. Goddard N Nov-99 14.41 12.22 2. Goddard N Dec-99 15.18 13.02 2. Goddard N Jan-00 15.27 13.1 2. Goddard N Feb-00 14.63 12.51 2. Goddard N Mar-00 13.18 11. Goddard N Apr-00 11.71 8.99 2. Goddard N May-00 9.75 6.31 3. Goddard N Jun-00 7.21 4.71 2.5
2000 9 Goddard N Jul-00 6.32 4.31 2. Goddard N Aug-00 8.92 6.64 2. Goddard N Sep-00 10.54 8.81 1. Goddard N Oct-00 12.81 10.9 1. Goddard N Nov-00 14.31 12.27 2. Goddard N Dec-00 15.27 13.13 2. Goddard N Jan-01 15.61 13.57 2. Goddard N Feb-01 14.86 12.99 1. Goddard N Mar-01 13.72 11.32 2.4
2001 6 Goddard N Apr-01 11.69 9.01 2. Goddard N May-01 9.22 6. Goddard N Jun-01 7.47 4.87 2.6
2001 9 Goddard N Jul-01 6.75 4.55 2. Goddard N Aug-01 8.59 6. Goddard N Sep-01 10.92 9.02 1. Goddard N Oct-01 12.84 10.49 2. Goddard N Nov-01 14.45 12.18 2. Goddard N Dec-01 15.36 13. Goddard N Jan-02 15.44 13.36 2. Goddard N Feb-02 14.37 12.35 2. Goddard N Mar-02 13.12 11.11 2. Goddard N Apr-02 11.69 9.13 2. Goddard N May-02 9.49 6.34 3. Goddard N Jun-02 6.53 4.23 2.3
2002 9 Goddard N Jul-02 5.96 3.98 1. Goddard N Aug-02 8.81 6.2 2. Goddard N Sep-02 10.78 8.69 2. Goddard N Oct-02 12.82 10.58 2. Goddard N Nov-02 14.46 12.21 2. Goddard N Dec-02 15.25 13.16 2. Goddard N Jan-03 15.49 13.36 2. Goddard N Feb-03 14.57 12.38 2. Goddard N Mar-.82 2. Goddard N Apr-03 11.77 9.05 2. Goddard N May-03 9.46 6.06 3.4
2003 8 Goddard N Jun-03 6.85 4.44 2. Goddard N Jul-03 6.15 4.01 2. Goddard N Aug-03 8.65 5.93 2. Goddard N Sep-03 10.29 8.5 1. Goddard N Oct-03 12.82 10.74 2. Goddard N Nov-03 14.03 12.19 1. Goddard N Dec-03 14.93 12.87 2. Goddard N Jan-04 15.05 12.93 2. Goddard N Feb-04 14.11 12.08 2. Goddard N Mar-04 12.58 10.86 1. Goddard N Apr-04 11.51 9.18 2. Goddard N May-04 9.6 6.43 3. Goddard N Jun-04 6.83 4.57 2. Goddard N Jul-04 6.05 4.35 1. Goddard N Aug-04 8.48 6.35 2. Goddard N Sep-04 10.65 8.9 1. Goddard N Oct-04 12.72 10.91 1. Goddard N Nov-04 13.66 11.84 1. Goddard N Dec-04 14.36 12.27 2. Goddard N Jan-05 14.74 12.67 2. Goddard N Feb-05 14.07 12.16 1. Goddard N Mar-05 12.99 10.83 2. Goddard N Apr-05 11.29 8.74 2. Goddard N May-05 8.93 5.81 3. Goddard N Jun-05 6.3 4.14 2. Goddard N Jul-05 5.57 4.03 1. Goddard N Aug-05 8.45 5.71 2. Goddard N Sep-05 10.47 8.73 1. Goddard N Oct-05 12.47 10.51 1. Goddard N Nov-05 13.6 11.57 2. Goddard N Dec-05 14.42 12.28 2. Goddard N Jan-06 14.43 12.44 1. Goddard N Feb-06 13.97 11. Goddard N Mar-06 12.62 10.39 2. Goddard N Apr-06 11.06 8.34 2. PRELIM N May-06 8.7 5.71 2. PRELIM N Jun-06 6.51 4.23 2. NRTSI-G N Jul-06 5.87 3.95 1. NRTSI-G N Aug-06 8.33 5.73 2. NRTSI-G N Sep-06 9.87 8.08 1. NRTSI-G N Oct-06 12.27 10.1 2. NRTSI-G N Nov-06 13.81 11.8 2. NRTSI-G N Dec-06 14.54 12.52 2. NRTSI-G N Jan-07 14.66 12.49 2. NRTSI-G N Feb-07 13.9 11.79 2. NRTSI-G N Mar-07 12.98 10.97 2. NRTSI-G N Apr-07 11.53 8.15 3. NRTSI-G N May-07 8.12 5.01 3. NRTSI-G N Jun-07 5.32 3.09 2.23
September 6, 2007 at 1:42 pm
Don Fontaine
I messed up the date column I added. I assumed that the data set was complete, but there are missing months. OOOPS
September 6, 2007 at 1:44 pm
Don Fontaine
The peak difference is always July.
September 6, 2007 at 1:47 pm
Don Fontaine
Notes at bottom of data sets:
Important Note: The “extent” column includes the area near the pole not
imaged by the sensor. It is assumed to be entirely ice covered with at
least 15% concentration. However, the “area” column excludes the area not
imaged by the sensor. This area is 1.19 million square kilometers for SMMR
(from the beginning of the series through June 1987) and 0.31 million
square kilometers for SSM/I (from July 1987 to present). Therefore, there
is a discontinuity in the “area” data values in this file at the June/July
1987 boundary.
September 6, 2007 at 3:42 pm
fergusbrown
You’ve been busy, Dom. Is that the 1987 question answered, then?
I’ll go to NSIDC and try to link to the data directly.
btw: what is the average difference between E & A? I’d guess around 15-20%.
Regards, and sorry for not replying sooner.